Depending upon particular cellular conditions, the tumor suppressor protein p53 induces growth arrest or mediates an apoptotic response. Inactivation of the apoptotic response, in particular, has been implicated in the process of oncogenesis as well as in the resistance of tumor cells to particular therapies. Since the DNA binding activity of p53 plays a role in each of the physiological responses to p53, the ability of p53 to select among various target genes to elicit a particular cellular response may be central to the regulation of its biological function. To date, the identification of a mechanism for the regulation of target gene selectivity by p53 has been elusive. The research that is proposed in this application is designed to test the hypothesis that the cellular response to p53 is determined by regulation of its target gene selectivity and that defects in the expression of particular target genes provide the molecular basis for defective p53-dependent apoptosis in tumor cells. Previous studies have shown differential p53-dependent regulation of a gene involved in growth arrest, the cyclin-dependent kinase inhibitor p21, as compared to one involved in apoptosis, bax. The molecular basis for this selectivity will be explored through three specific aims. First, since p53 binding sites in the p21 and bax promoters require additional gene-specific elements for robust p53-dependent transcriptional regulation, studies are proposed to elucidate the underlying mechanisms by which such cis-acting elements cooperate with particular p53 binding sites to affect transcription in a gene-specific manner. Two tumor-derived mutants had previously been identified which retain the ability to induce growth arrest but had selectively lost the capacity to trigger apoptosis. While the two mutants were capable of upregulating the p22 promoter, both showed defective activation of the bax promoter. In the second aim the activity of these two mutant p53 proteins will be examined to elucidate the basis for these promoter-specific effects. Three tumor cell lines have been identified in which p.53 efficiently upregulated the p21 promoter but showed defective activation of bax. The third aim is to determine the molecular basis for this differential regulations. The optimal therapeutic response to DNA damage caused by many chemotherapeutic agents is apoptosis rather than cell cycle arrest. Elucidating the molecular mechanisms that are responsible for the ability of p53 to trigger apoptosis versus arrest may lead to more effective therapeutic intervention and a way to overcome the chemotherapeutic-resistant phenotype found in many tumors.